1. Field of the Invention
The present invention relates to an elastic conductive resin, and an electronic device in which an electronic part and a substrate are electrically connected to each other via elastic conductive bumps made of the elastic conductive resin.
2. Discussion of the Background
Published Japanese patent application 10-242616 describes an integrated circuit (IC) package in which a circuit board and an IC chip are connected to each other with high reliability without filling a sealing resin into a gap between the circuit board and the IC chip. Specifically, as illustrated in FIGS. 1A and 1B, a plurality of bump seats 2 are provided on a bottom part of an IC chip 1. Conductive adhesive agents 3 connect the bump seats 2 to elastic conductive resin bumps 4, respectively. Generally, a conductive bump is a projecting electrode formed at an electrode part of a chip for wireless bonding in an integrated circuit. The elastic conductive resin bump 4 is made of a material in which a conductive powder (e.g., a filler) is mixed with a silicone resin. The volume ratio between the silicone resin and the conductive powder is two to one. The conductive powder is, for example, a copper powder subjected to gold plating and having a diameter in a range of 180 μm to 200 μm. The elastic conductive resin bump 4 adsorbs stresses due to heat and mechanical distortions. A plurality of part mounting seats 6 are provided on an upper part of a circuit board 5 and are electrically connected to the elastic conductive resin bumps 4, respectively.
Another piece of background art with regard to an electronic device using an elastic conductive resin is Published Japanese patent application 10-256304. Specifically, in a semiconductor device illustrated in FIG. 2, a conductive adhesive agent 15 having rubbery elasticity is applied to each of projecting electrodes 13 provided on a functional surface of a semiconductor integrated circuit chip 11. The conductive adhesive agent 15 is made of conductive particles and a heat-curing resin. The semiconductor integrated circuit chip 11 is mounted on an insulating substrate 12 while positioning the projecting electrodes 13 and substrate electrodes 14 provided on the insulating substrate 12. A sealing resin 16 is filled into a gap between the semiconductor integrated circuit chip 11 and the insulating substrate 12 and is cured and contracts. The projecting electrodes 13 and the substrate electrodes 14 are electrically connected to each other via the conductive adhesive agent 15.
According to Published Japanese patent application 10-256304, after the conductive adhesive agent 15 is cured, even if a shearing stress is imposed on the conductive adhesive agent 15 caused by the difference of the thermal expansion coefficient between the semiconductor integrated circuit chip 11 and the insulating substrate 12 as they are cooling, the conductive adhesive agent 15 does not break and separate from the substrate electrodes 14 because the conductive adhesive agent 15 moderates the shearing stress. Further, because the conductive adhesive agent 15 does not prevent a longitudinal stress caused by the cured and contacted sealing resin 16, the projecting electrodes 13 and substrate electrodes 14 are press-contacted to each other through the conductive particles in the conductive adhesive agent 15, thereby achieving a good electrical connection.
Generally, when mounting an IC chip on a substrate, the IC chip and the substrate are connected to each other by solder. In this case, under the condition of temperature changes, stresses due to the difference in thermal expansion coefficient between the IC chip and the substrate occur at the connection part between the IC chip and the substrate. Therefore, to avoid trouble such as breakage of the connection part, a resin is used for reinforcing the connection part.
In an electronic device having multiple pins and a large size, an IC chip and a substrate are connected to each other by using a conductive adhesive agent having rubbery elasticity. In this type of the electronic device, to ensure moderation in the stress due to the difference in thermal expansion coefficient between the IC chip and the substrate, a sealing resin is necessary as a reinforcement for filling a gap between the IC chip and the substrate.
In an electronic device in which a plurality of bumps made of an elastic conductive resin are formed on an IC chip, and the IC chip and a substrate are electrically connected to each other by press-contacting the bumps with electrodes provided on the substrate; the bumps need to contact with the electrodes formed on the substrate, especially, in an area array state, with a large press-contacting force. Further, if the bumps have uneven height, press-contacting forces are not equally exerted on the bumps, thereby causing an unstable electrical connection between the IC chip and the substrate.
In the background electronic devices, an elastic conductive resin used for forming bumps often includes a silicone resin having rubber-like elasticity and containing spherical conductive particles or flake conductive fillers. To obtain high conductivity by using the spherical conductive particles or flake conductive fillers, the compounding ratio of the flake conductive filler in the silicone resin needs to be increased. However, if the amount of the flake conductive filler increases in the silicone resin, the silicone resin hardens due to the flake conductive filler. In this condition, the silicone resin cannot exhibit a good rubber-like elasticity property. If an IC chip and a substrate are electrically connected to each other via the bumps made of the silicone resin without having a sufficient rubber-like elasticity property, the bumps need to contact with electrodes formed on the substrate with a large press-contacting force. Further, the bumps need to be controlled with high accuracy to have equal height.
Therefore, it is desirable to provide an elastic conductive element, such as, an elastic conductive bump, that has high deformation ability against a compression force and high conductivity.